SFH4045_ データシートの表示（PDF） - OSRAM GmbH
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SFH4045_ Datasheet PDF : 14 Pages
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Fig. 4: Principle of projector-based
touchscreen realization. The depicted
system works with diffuse illumination (DI).
2.4 Projector-based Touchscreens
Another group of systems are based on a
projector concept. Due to the setup their
main application is in large screens for
overview or presentation purposes. The
principle of such a technique is presented in
Fig. 4.
Usually the visible image is projected from
the backside onto a diffuse screen. One or
several IR sensitive cameras are mounted
behind the screen to monitor the reflected IR
image of the screen.
To illuminate the screen with IR radiation
there are various options. One makes use of
diffuse illumination (DI) from IR-sources
behind the screen. If a stylus or finger
touches the screen, a reflection occurs and
the IR camera detects the bright spot.
2.5 Projector-based with FTIR
Fig. 5 presents a similar version which works
on the principle of frustrated total internal
reflection (FTIR).
This setup uses the waveguide properties of
e.g. the acrylic glass as a part of the screen
to distribute the IR radiation. Usually a
pressure sensitive polymer layer is added on
top to display the projected image, as acrylic
glass is almost transparent to the visible
image. IR light is coupled into the acrylic
Fig. 5: Principle of a FTIR-based touch
detection system. Different IRED coupling
options are sketched.
waveguide usually by several IREDs located
on all sides of the screen. The light is
captured inside the waveguide by total
internal reflection.
If pressure or a touch is applied on the
polymer/acrylic surface due to a stylus or an
object (e.g. finger) light is coupled by FTIR
into the polymer (or into the finger if no
polymer is used), from where it is scattered
and remitted towards the IR sensitive
camera located in the rear part of the
screen. This technique is desired for
applications where IR emission through the
screen should be avoided, e.g. in touch
screens used in TV studios to avoid
interferences or saturation of TV camera
pictures by IR light.
It should be mentioned that FTIR combined
with camera sensors is also used in the
biometrics industry, most notably in
fingerprint scanning applications.
2.6 In-Cell Optical Sensing
The in-cell optical sensing principle is an
integrated solution. Inside each pixel cell in
a LCD display there is typically a
phototransistor integrated. The principle
works without a designated light source. In a
bright environment the phototransistor sees
the shadow of the finger tip, whereas in a
dark or dim lit ambience the reflections of
the backlight generates the signal. The
absence of an active illumination is also the
drawback of this principle, especially a black
screen in dark environments.
August 13, 2010
page 4 of 14
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